VALENCIA, Spain — They are seen as the gurus of global warming, and their reports are accepted almost as the gospel of climate science. Esteem for the panel of scientists was immortalized when it shared this year's Nobel Peace prize.

But experts and the scientists themselves acknowledge the reports are conservative and have a poor track record of predictions.

As the Intergovernmental Panel on Climate Change meets in this Mediterranean coastal city to finish its fourth report in two decades, it must decide whether it will produce a fifth.

"Next year would complete 20 years of the IPCC," said chairman Rajendra Pachauri. "That clearly is a point where we should carry out deep and detailed introspection on what we have achieved, what we could have achieved further, and how we might be able to ensure achievement in the future."

The future of the panel, which shared the Nobel with former Vice President Al Gore, is an agenda item at the weeklong meeting that wraps up Saturday with the release of its latest climate change assessment.

The panel, a loose organization of more than 2,000 scientists who submit their own research and review the work of others, has issued extensive reports on global warming every five or six years since it was created under the auspices of two U.N. bodies. Each report includes a brief Summary for Policymakers, which is approved by government representatives.

Driven by peer reviewsThe assessment reports have won accolades as the most authoritative compilation of climate science available, largely because of the rigorous process of peer review. Even if the data produced by different researchers may conflict, the science is deemed to be solid.

But its thoroughness is also a shackle that condemns it to lag behind the latest research. It usually takes the collaborative efforts of dozens of IPCC authors about two years to compile a report and takes at least a year for a scientific paper to be reviewed.

In its assessment this year, the IPCC has reported with near certainty that human activity is causing temperatures to rise and changing weather patterns. Sea levels are rising, storms are growing more fierce, deserts are spreading and glaciers are melting, it said. The panel predicts that millions of poor people will suffer from hunger, thirst, floods and disease unless drastic action is taken. But it also says the tools are available, or soon will be, to greatly slow global warming, soften the impact and help nations adapt to changing conditions.

As grim as that may look, the reality may be much worse.

"We need to understand that the worst impacts in the report may not in fact be the worst that will happen, or the worst that appear possible," said Peter Altman, the climate policy project manager for the National Environmental Trust, a Washington lobby.

"What's in the report now is scary enough. But in most of the predictions the IPCC has made, just about everything is happening faster and more intensely than we thought," he said. "This issue is not being overstated. If anything, it is being understated."

Think tanks cite shortfallA joint report this month by two U.S. security institutes said they compared predictions of climate change by the panel and other researchers in the last two decades with changes that actually occurred, and found the scientists had consistently fallen short.

Part of the reason was the lack of data, but it also could be that the scientists shied away from controversy and wanted to avoid being discredited as "alarmists," said the paper by the Center for Strategic International Studies and the Center for a New American Security.

The scientists see their caution as a strength rather than a shortcoming.

"The process tends to lead to a fairly conservative outcome," said Bert Metz, one of the IPCC's lead authors, because the scientists will wait for verification of research before pronouncing a possible trend.

"By not using an alarmist or advocatist tone, you in the end gain much more credibility," said Metz, of the Netherlands Environmental Assessment Agency.

The World Wide Fund for Nature, or WWF International, which is invited with other environmental groups to observe IPCC meetings, agrees the IPCC process is cumbersome and late.

"Climate change is going faster than our worst-case scenarios of five or six years ago," said WWF's climate specialist Hans Verolme.

Still, he said, it would be difficult for the panel to issue more frequent reports to try to keep pace with the research without sacrificing its scientific integrity. "I wouldn't know how to speed up the process," he said.

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